1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and a driving method thereof adapted to reduce the generation of heat and power consumption of a data driving circuit, prevent DC image sticking and flicker, and prevent degradation of picture quality when displaying data of weakness patterns.
2. Discussion of the Related Art
A liquid crystal display is adapted to display images by controlling the light transmittance of liquid crystal cells in response to a video signal. A liquid crystal display of an active matrix type actively controls data by switching a data voltage applied to the liquid crystal cells using a thin film transistor (TFT) formed at every liquid crystal cell Clc, as shown in FIG. 1, thereby improving the picture quality of a motion image. As shown in FIG. 1, reference label “Cst” denotes a storage capacitor for sustaining the data voltage charged in the liquid crystal cell “Clc,” “D1” denotes a data line through which the data voltage is supplied, and “G1” denotes a gate line through which a scan voltage is supplied.
The liquid crystal display is driven according to an inversion method in which a polarity is inverted between neighboring liquid crystal cells and a polarity is inverted whenever a frame period is shifted in order to reduce a direct current (DC) offset component and the degradation of liquid crystals. However, the swing width of the data voltage supplied to the data lines whenever the polarity of the data voltage is shifted increases, thereby generating a large amount of current in a data driving circuit. Thus, the problems that arise include increase in heat generating temperature of the data driving circuit and sharp increase in power consumption of the data driving circuit. In order to reduce the swing width of the data voltage supplied to the data lines thereby decreasing the heat generating temperature and power consumption of the data driving circuit, a charge sharing circuit or a precharge circuit is adopted in the data driving circuit. However, the resulting effects are generally not satisfactory.
Further, if the polarity of the data voltage is inverted according to the inversion method, the charging amount of a liquid crystal cell charged with the data voltage of a positive polarity is different from that of a liquid crystal cell charged with the data voltage of a negative polarity. Thus, there is a problem that the picture quality is degraded. For example, as shown in FIG. 2, assuming that a liquid crystal cell is charged with the data voltage of a positive polarity and then with the data voltage of a negative polarity for representing the same gray level as that of the data voltage of the positive polarity, the liquid crystal cell maintains a voltage Vp(+) whose absolute value voltage may be lowered by as much as ΔVp due to parasitic capacitance of the TFT after being charged with the data voltage of the positive polarity. Then, the liquid crystal cell maintains a voltage Vp(−) whose absolute value voltage may be increased by as much as ΔVp due to parasitic capacitance of the TFT after being charged to the data voltage of the negative polarity. Accordingly, a liquid crystal cell of a normally black mode liquid crystal display has light transmitted therethrough with a higher light transmittance when being charged to the data voltage of a negative polarity for representing the same gray level as that of the data voltage of a positive polarity than that of the data voltage of the positive polarity. In the normally black mode, the higher the voltage charged with a liquid crystal cell, the higher the light transmittance of the liquid crystal cell. Further, a liquid crystal cell of a normally white mode liquid crystal display has light transmitted therethrough with a lower light transmittance when charged with the data voltage of a negative polarity for representing the same gray level as that of the data voltage of a positive polarity than that of the data voltage of the positive polarity. In the normally white mode, the higher the voltage charged in a liquid crystal cell, the lower the light transmittance of the liquid crystal cell.
Further, low picture quality may result on a liquid crystal display when a particular data pattern of a specific picture having a particular polarity pattern of a data voltage applied to liquid crystal cells and the gray levels of data is displayed. Representative factors that degrade the picture quality include a phenomenon in which greenish tint is generated on a display screen and flicker in which the luminance of a screen is shifted periodically.
For example, greenish tint may be generated in a display image when a liquid crystal display is driven according to a vertical 2-dot and horizontal 1-dot inversion method (V2H1) in which the polarity of a data voltage charged in the liquid crystal cells every vertical 2-dot (or 2 liquid crystal cells) is inverted and the polarity of a data voltage charged in the liquid crystal cells every horizontal 1-dot (or 1 liquid crystal cell) is inverted and the gray levels of data supplied to odd pixels are white gray levels and the gray levels of data supplied to even pixels are black gray levels within a one frame period, as shown in FIG. 3. In other words, in first, second, fifth, and sixth lines L1, L2, L5, and L6, the data voltage of all green (G) data, which have the greatest influence on the luminance compared to red (R), green (G), and blue (B) data, has a negative polarity. Consequently, greenish tint is generated in the first, second, fifth, and sixth lines of this image. This greenish phenomenon is generated because the green (G) data is biased toward any one polarity. FIG. 4 illustrates another example of when greenish tint may be generated in an image. As shown in FIG. 4, greenish tint is generated in a display image when a liquid crystal display is driven according to a vertical 2-dot and horizontal 1-dot inversion method (V2H1), and the gray levels of data supplied to odd subpixels are white gray levels and the gray levels of data supplied to even subpixels are black gray levels.
Another picture degrading phenomenon occurs when a liquid crystal display is driven according to a vertical 1-dot and horizontal 1-dot inversion method (V1H1) in which the polarity of a data voltage is inverted every vertical 1-dot and horizontal 1-dot such that the polarities of data voltages charged in adjacent liquid crystal cells in vertical and horizontal directions are inverted and the data voltages include a data voltage of a white gray level and a data voltage of a black gray level alternately disposed every one subpixel within a one frame period, as shown in FIG. 5. In this instance, a flicker phenomenon in which the luminance of a display image is shifted every frame period is generated. In other words, all of the data voltages of the white gray levels have a positive polarity and all of the data voltages of the white gray levels in a next frame have a positive polarity within one frame period. Consequently, the luminance of a display image is changed every frame period.
An image in which a white gray level and a black gray level are alternately arranged periodically as shown in FIGS. 3 to 5 is referred to as an image containing a “weakness pattern” since such polarity and gray level patterns degrade the picture quality of a display image.
Another picture degrading phenomenon occurs when any one of two polarities of a data voltage is supplied to a liquid crystal display panel dominantly for a long time, thereby generating image sticking phenomenon. This image sticking is referred to as “DC image sticking” because the phenomenon is generated by a voltage of the same polarity being repeatedly charged to a liquid crystal cell. One example of when DC image sticking occurs is when data voltages of an interlaced video signal are supplied to a liquid crystal display. Data according to an interlace method (hereinafter, referred to as “interlace data”) is characterized by data voltages of an image to be displayed on a liquid crystal cell being applied only to odd horizontal lines in odd frame periods and only to even horizontal lines in even frame periods.
FIG. 6 shows a waveform illustrating an example of the interlace data supplied to the liquid crystal cell Clc. For purposes of example, the liquid crystal cell Clc to which the data voltage is supplied, as shown in FIG. 6, is a liquid crystal cell arranged in odd horizontal lines. As shown in FIG. 6, the liquid crystal cell Clc is supplied with a positive voltage during an odd frame period and is supplied with a negative voltage during an even frame period. In the interlace method, a high positive data voltage is supplied to the liquid crystal cell Clc arranged in the odd horizontal line only during odd frame periods. For this reason, the positive data voltage is dominant compared with the negative data voltage as indicated by waveforms within the box shown in FIG. 6 over four frame periods. Consequently, DC image sticking is generated.
FIG. 7 shows an image illustrating an experiment result of DC image sticking that appears due to the interlace data. If an original image as shown on the left side of FIG. 7 is supplied to a liquid crystal display panel for a certain period of time using the interlace method, the amplitude of a data voltage whose polarity is changed every frame period varies between an odd frame and an even frame. As a result, if data voltages of intermediate gray levels (e.g., 127 gray levels) are supplied to all the liquid crystal cells Clc of the liquid crystal display panel after the original image shown on the left side of FIG. 7 is supplied, DC image sticking in which the pattern of the original image appears faintly occurs as shown on the right side of FIG. 7.
Another example of when DC image sticking occurs is when the same image is moved or scrolled at a constant speed. The voltage of the same polarity is repeatedly accumulated in the liquid crystal cell Clc based on the size of the figure being scrolled and the scroll speed (i.e., moving speed), thereby resulting in DC image sticking. FIG. 8 shows an image illustrating an experimental result of DC image sticking that appears when oblique patterns and character patterns are moved at a constant speed.